Steel Sheet Excellent In Fine Blanking Performance and Manufacturing Method of the Same

ABSTRACT

A steel sheet excellent in FB performance-and also excellent in fabrication performance after FB working and a manufacturing method of the same are provided. The steel sheet is a steel sheet having a composition containing from 0.1 to 0.5% of C, not more than 0.5% of Si and from 0.2 to 1.5% of Mn in terms of % by mass, with P and S being adjusted at proper ranges, and having a structure in which a ferrite has an average grain size of from 1 to 10 μm, cementite has a spheroidization ratio of 80% or more, and of the cementite, an amount S gb  of ferrite intergranular cementite which is defined by the following expression (1): S gb (%)={S on /(S on +S in )}×100 (wherein S on  represents a total occupied area of cementite present on the ferrite grain boundary of the cementite present per unit area; and Sn in  represents a total occupied area of cementite present in a ferrite grain of the cementite present per unit area) is 40% or more. In this way, the steel sheet becomes a steel sheet excellent in FB performance, mold life and performance after FB working.

TECHNICAL FIELD

The present invention is concerned with a steel sheet suitable forapplications to automobile parts or the like and in particular, relatesto a steel sheet excellent in fine blanking performance suitable for theuses to which fine blanking working (hereinafter also referred to as “FBworking”) is applied.

BACKGROUND ART

In manufacturing complicated mechanical parts, from the viewpoints of animprovement in dimension precision, a reduction in manufacturingprocess, and the like, it is known that fine blanking working is anextremely advantageous working method as comparing with machiningworking.

In usual blanking working, a tool-to-tool clearance is fromapproximately 5 to 10% of a thickness of a metal sheet as a material tobe blanked. However, the fine blanking working differs from the usualblanking working and is a blanking working method of not only setting upthe tool-to-tool clearance extremely small as substantially zero(actually, not more than approximately 2% of the thickness of the metalsheet as a material to be blanked) but also making a compression stressact on a material in the vicinity of a tool cutting blade. Then, thefine blanking working has the following characteristic features.

-   -   (1) The generation of a crack from the tool cutting blade is        inhibited, and a fracture surface seen in usual blanking working        becomes substantially zero, whereby a smooth worked surface        (blanked end surface) in which the worked surface is a        substantially 100% shear surface is obtained.    -   (2) The dimensional precision is good.    -   (3) A complicated shape can be blanked by one process.

However, in the fine blanking working, a working ratio which thematerial (metal sheet) receives is extremely severe. Also, in the fineblanking working, since the working is carried out at a tool-to-toolclearance of substantially zero, there is involved a problem that a loadto a mold becomes excessive so that a mold life is shortened.

For that reason, materials to which the fine blanking working is appliedare required to not only have excellent fine blanking performance butalso prevent a reduction in mold life.

In response to these requirements, for example, Patent Document 1proposes a high carbon steel sheet excellent in fine blankingperformance, which has a composition containing from 0.15 to 0.90% byweight of C, not more than 0.4% by weight of Si and from 0.3 to 1.0% byweight of Mn, has a microstructure with a cementite having aspheroidization ratio of 80% or more and an average grain size of from0.4 to 1.0 μm scattered in a ferrite matrix and has a notch tensileelongation of 20% or more. According to a technology described in PatentDocument 1, it is described that the fine blanking performance isimproved and that the mold life is also improved.

However, the high carbon steel sheet described in Patent Document 1involved a problem that fabrication performance after the fine blankingworking is inferior.

Also, Patent Document 2 proposes a steel sheet for fine blankingprepared by applying proper hot rolling to a billet containing from 0.08to 0.19% of C and proper amounts of Si, Mn and Al and containing from0.05 to 0.80% of Cr and from 0.0005 to 0.005% of B into a steel sheet.It is described that the steel sheet described in Patent Document 2 is asteel sheet which is low in a yield strength, high in an impact value,excellent in fine blanking performance, high in an n-value in a lowstrain region, excellent in combined formability and excellent inquenching property at short-time rapid heating. However, Patent Document2 does not show concrete evaluation regarding the fine blankingperformance. Also, the steel sheet described in Patent Document 2involved a problem that fabrication performance after the fine blankingworking is inferior.

Also, Patent Document 3 proposes a high carbon steel sheet excellent inflow forming and fine blanking working, which has a compositioncontaining from 0.15 to 0.45% of C, with the contents of Si, Mn, P, S,Al and N being adjusted at proper ranges and has a structure having afractional ratio of (pearlite+cementite) of not more than 10% and anaverage grain size of ferrite grain of from 10 to 20 μm. It is describedthat the high carbon steel sheet described in Patent Document 3 isexcellent in fine blanking performance and is improved in mold life inthe fine blanking working. However, the high carbon steel sheetdescribed in Patent Document 3 involved a problem that fabricationperformance after the fine blanking working is inferior.

Furthermore, it is hard to say that all of the steel sheets described inPatent Document 1, Patent Document 2 and Patent Document 3 are notprovided with satisfactory and thorough fine blanking performance in thefine blanking working under a recent severe working condition. Also,problems that the mold life is not thoroughly improved and thatfabrication performance after the fine blanking working is inferiorstill remained.

At the beginning, the fine blanking working had been applied to parts towhich working is not applied after fine blanking working even among gearparts and the like. However, recently, the application of fine blankingworking to automobile parts (for example, reclining parts) tends toexpand, and its application to parts which require stretch flangingworking, bulging, etc. is investigated. For that reason, steel sheetswhich are not only excellent in fine blanking performance but alsoexcellent in fabrication performance after fin blanking working instretch flanging working, bulging, etc. are eagerly desired asautomobile parts.

As a technology for improving stretch flanging workability, there havebeen made a number of proposals up to date. For example, Patent Document4 proposes a wear resistant hot rolled steel sheet excellent in stretchflanging property, which has a composition containing from 0.20 to 0.33%of C, with the contents of Si, Mn, P, S, sol. Al and N being adjusted atproper ranges and further containing from 0.15 to 0.7% of Cr and has aferrite-bainite mixed structure which may contain pearlite. In the hotrolled steel sheet described in Patent Document 4, it is described thatby taking the foregoing structure, a hole expansion ratio becomes high,whereby the stretch flanging property is improved. Also, Patent Document5 proposes a high carbon steel sheet excellent in stretch flangingproperty, which has a composition containing from 0.2 to 0.7% of C andhas a structure in which a cementite average particle size is 0.1 μm ormore and less than 1.2 μm and a volume ratio of a cementite-free ferritegrain is not more than 15%. In the high carbon steel sheet described inPatent Document 5, it is described that the generation of a void on anend surface at the time of blanking is inhibited, that the growth of acrack in hole expansion working can be made slow and that the stretchflanging property is improved.

Also, Patent Document 6 proposes a high carbon steel sheet excellent inblanking performance and quenching property, which has a compositioncontaining 0.2% or more of C and has a structure composed mainly offerrite and a cementite and having a cementite particle size of not morethan 0.2 μm and a ferrite grain size of from 0.5 to 1 μm. It isdescribed that according to this, both blanking performance andquenching property which are determined by a burr height and mold lifeare improved.

-   -   Patent Document 1: JP-A-2000-265240    -   Patent Document 2: JP-A-59-76861    -   Patent Document 3: JP-A-2001-140037    -   Patent Document 4: JP-A-9-49065    -   Patent Document 5: JP-A-2001-214234    -   Patent Document 6: JP-A-9-316595

DISCLOSURE OF THE INVENTION

However, all of the technologies described in Patent Document 4 andPatent Document 5 are those made on the assumption that the conventionalblanking working is applied but not those made taking into considerationthe application of fine blanking working in which the clearance issubstantially zero. Accordingly, it is difficult to ensure similarstretch flanging property after the severe fine blanking working, andeven when the stretch flanging property can be ensured, there isencountered a problem that the mold life is short.

Also, in the technology described in Patent Document 6, it is necessarythat the ferrite grain size is in the range of from 0.5 to 1 μm; and itis difficult to stably manufacture a steel sheet having such a ferritegrain size on an industrial scale, resulting in a problem that theproduct yield is reduced.

In view of the foregoing problems of the conventional technologies, theinvention has been made, and an object thereof is to provide a steelsheet excellent in fine blanking performance and also excellent infabrication performance after fine blanking working and a manufacturingmethod of the same.

In order to achieve the foregoing object, the present inventors madeextensive and intensive investigations regarding influences of ametallographic structure against fine blanking performance (hereinafterabbreviated as “FB performance”), especially influences againstmorphology and distribution state of ferrite and a cementite.

As a result, it has been found that the FB performance and the mold lifeare closely related with a particle size of a cementite present in aferrite grain and a ferrite grain size. Then, it has been found thatwhen a raw steel material having a composition of a prescribed range isformed into a hot rolled steel sheet having a substantially 100%pearlite structure by making a finish rolling condition of hot rollingand a condition of subsequent cooling proper, which is then subjected tohot rolling annealing under a proper condition, thereby converting themetallographic structure into a (ferrite+cementite) (sphericalcementite) structure in which a cementite amount in ferrite grain iscontrolled such that an average ferrite grain size is not more than 10μm, a spheroidization ratio of a cementite is 80% or more and a ratio ofan area of a cementite present on a ferrite grain boundary to an area ofthe whole of cementites is 40% or more, the FB performance and the moldlife are remarkably improved. Also, it has been newly found that whenthe cementite amount in ferrite grain is controlled, the fabricationproperty after the FB working is remarkably improved.

In the FB working, the material is worked in a state of zero clearanceand compression stress. For that reason, after receiving largedeformation, a crack is generated in the material. When a number ofcracks are generated during large deformation, the FB performance islargely reduced. In order to prevent the generation of a crack, it issaid that spheroidization of a cementite or miniaturization of acementite particle size is important. However, in the FB working, in thecase where even a 100% spheroidized fine cementite is present in theferrite grain, the generation of a fine crack is unavoidable. For thatreason, the present inventors thought that in the case where stretchflanging working is further applied after the FB working, fine cracksgenerated at the time of the FB working are connected to each other,leading to a reduction in the stretch flanging property. Also, withrespect to the mold life, the present inventors assumed that when anumber of cementites are present in the ferrite grain, wear of a cuttingblade is accelerated, leading to a reduction in the mold life.

First of all, the experimental results on a basis of which the inventionhas been made are described.

A high steel slab (corresponding to S35C) containing 0.34% of C, 0.2% ofSi and 0.8% of Mn in terms of % by mass was heated at 1,150° C. and thensubjected to hot rolling consisting of rough rolling of 5 passes andfinish rolling of 7 passes, thereby preparing a hot rolled steel sheethaving a thickness of 4.2 mm. Incidentally, a rolling terminationtemperature was set up at 860° C.; a coiling temperature was set up at600° C.; and after the finish rolling, the steel sheet was cooled whilechanging a cooling rate from 5° C./s to 250° C./s. Incidentally, in thecase where cooling (forced cooling) other than air cooling was carriedout, a cooling stopping temperature was set up at 650° C. Subsequently,the hot rolled steel sheet was subjected to pickling and then to batchannealing (at 720° C. for from 5 to 40 hours) as hot rolled sheetannealing. With respect to the steel sheet to which the hot rolled sheetannealing had been thus applied, not only its metallurgical structurewas observed, but also its FB performance was evaluated.

In the observation of the metallurgical structure, a specimen wascollected from the obtained steel sheet; a cross section parallel to arolling direction of the subject specimen was polished and corroded withnital; and with respect to a position of ¼ of the sheet thickness, themetallurgical structure was observed by a scanning electron microscope(SEM), thereby measuring a ferrite grain size and a spheroidizationratio of a cementite.

With respect to the ferrite grain size, an area of each ferrite grainwas measured, and a circle-corresponding size was determined from theresulting area and defined as a grain size of each ferrite grain. Thethus obtained respective ferrite grain sizes were arithmeticallyaveraged, and its value was defined as a ferrite average grain size ofthat steel sheet. Incidentally, the number of measured ferrite grainswas 5,000 for each.

Also, a maximum length a and a minimum length b of each cementite weredetermined in each field of the structure observation (magnification:3,000 times) by using an image analyzer; its ratio a/b was computed; andthe number of cementite grains with a/b of not more than 3 was expressedby a proportion (%) against the total number of measured cementites,thereby defining it as a spheroidization ratio (%) of cementite.Incidentally, the number of measured cementites was 9,000 for each.

Also, in each field of the structure observation, a cementite present onthe ferrite grain boundary and a cementite present in the ferrite grainwere discriminated from each other; with respect to the cementitespresent per unit area, an occupied area S_(on) of a cementite present onthe ferrite grain boundary and an occupied area S_(in) of a cementitepresent in the ferrite grain were measured by using an image analyzer;and an amount (S_(gb)) of a ferrite intergranular cementite as definedby the following expression:

S _(gb)(%)={S _(on)/(S _(on) +S _(in))}×100

was computed. Incidentally, the area of the cementite particle wasmeasured in 30 fields (magnification: 3,000 times) for each.

Also, a specimen (size: 100×80 mm) was collected from the obtained steelsheet and subjected to a fine blanking test (FB test). The FB test wascarried out by blanking a sample having a size of 60 mm×40 mm (cornerradius R: 10 mm) from the specimen by using a 110 t hydraulic pressmachine under a lubricious condition of a clearance of 0.060 mm (1.5% ofthe sheet thickness) and a working pressure of 8.5 tons. With respect toan end surface (blanked surface) of the blanked sample, a surfaceroughness (ten-point average roughness Rz) was measured, therebyevaluating the FB performance. Incidentally, with respect to thespecimen, in order to eliminate influences of a deviation in sheetthickness against the clearance, the both surfaces were equally groundin advance, thereby regulating the sheet thickness at 4.0±0.010 mm.

With respect to the measurement of the surface roughness, as illustratedin FIG. 4, in each of four end surfaces (sheet thickness surfaces) otherthan R parts, a region within a range of from 0.5 mm to 3.9 mm of thesurface in the punch side in the sheet thickness direction and 10 mm inparallel to the surface (X direction) was scanned 35 times at a pitch of100 μm in the sheet thickness direction (t direction) by using a contactprobe profilometer, and a surface roughness Rz in each scanning line wasmeasured according to JIS B 0601-1994. Furthermore, with respect to thesurface roughness Rz on the measured surface, Rzs in the respectivescanning lines were summed up, and an average value thereof wasemployed. The four end surfaces were measured in the same method asdescribed above, and an average surface roughness Rz ave (μm) definedaccording to the following expression: Rz ave=(Rz 1+Rz 2+Rz 3+Rz 4)/4(wherein Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface)was computed.

In general, the case where the appearance of the fracture surface on theblanked surface is not more than 10% is defined as “excellent in FBperformance”. However, in the invention, the case where the averagesurface roughness Rz ave is small as 10 μm or less is defined as“excellent in FB performance”. Incidentally, in the case of measuring asurface roughness of a specimen having a sheet thickness different fromthe foregoing, the measurement may be carried out by repeatedlyperforming scanning in a pitch of 100 μm in a sheet thickness directionin a region within a range of approximately {(sheet thickness (mm))−0.1mm} in the sheet thickness direction of 0.5 mm from the surface and 10mm in parallel to the surface to determine Rz on each surface, therebydetermining Rz ave from Rzs of the respective surfaces.

The obtained results are shown in FIGS. 1 and 2.

From a relationship between an average surface roughness Rz ave and aspheroidization ratio of a cementite as shown in FIG. 2, it is notedthat when the spheroidization ratio is 80% or more, Rz ave is not morethan 10 μm, and the FB performance is abruptly improved. Incidentally,the data shown in FIG. 2 is concerned with the case where the averageferrite grain size is from approximately 3 to 8 μm. Furthermore, it wasacknowledged that when the spheroidization ratio is 80% or more and theamount of an intergranular cementite increases, Rz ave becomes smallerand the FB performance is remarkably improved. From a relationshipbetween the surface roughness (average surface roughness: Rz ave) andthe amount (S_(gb)) of a ferrite intergranular cementite as shown inFIG. 1, when a proportion of the intergranular cementite of thecementites increases such that the amount of a ferrite intergranularcementite is 40% or more, Rz ave is not more than 10 μm and the FIBperformance is abruptly improved.

As a result of further extensive and intensive investigations on thebasis of the foregoing knowledge, the invention has been accomplished.That is, the gist of the invention is as follows.

(1) A steel sheet excellent in fine blanking performance, which ischaracterized by having a composition containing from 0.1 to 0.5% of C,not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% ofP and not more than 0.02% of S in terms of % by mass, with the remainderbeing Fe and unavoidable impurities and having a structure mainlycomposed of ferrite and cementites, wherein the foregoing ferrite has anaverage grain size of from 1 to 10 μm, the foregoing cementite has aspheroidization ratio of 80% or more, and of the foregoing cementites,an amount S_(gb) of a ferrite intergranular cementite which is an amountof a cementite present on a crystal grain boundary of ferrite and whichis defined by the following expression (1) is 40% or more:

S _(gb)(%)={S _(on)/(S _(on) +S _(in))}×100  (1)

(wherein S_(on) represents a total occupied area of a cementite presenton the ferrite grain boundary of the cementites present per unit area;and S_(in) represents a total occupied area of a cementite present in aferrite grain of the cementites present per unit area.)(2) The steel sheet as set forth in (1), which is characterized in thatthe cementite present on the crystal grain boundary of the foregoingferrite has an average particle size of not more than 5 μm.(3) The steel sheet as set forth in (1) or (2), which is characterizedin that in addition to the foregoing composition, the compositionfurther contains not more than 0.1% of Al in terms of % by mass.(4) The steel sheet as set forth in any one of (1) to (3), which ischaracterized in that in addition to the foregoing composition, thecomposition further contains one or two or more members selected fromnot more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5%of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in termsof % by mass.(5) A manufacturing method of a steel sheet excellent in fine blankingperformance including successively applying hot rolling by heating androlling a raw steel material to form a hot rolled sheet and hot rolledsheet annealing by applying annealing to the subject hot rolled sheet,which is characterized in that the foregoing raw steel material is a rawsteel material having a composition containing from 0.1 to 0.5% of C,not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% ofP and not more than 0.02% of S in terms of % by mass, with the remainderbeing Fe and unavoidable impurities; and the foregoing hot rolling is atreatment in which a termination temperature of finish rolling is set upat from 800 to 950° C., after completion of the subject finish rolling,cooling is carried out at an average cooling rate of 50° C./s or more,the subject cooling is stopped at a temperature in the range of from 500to 700° C., and coiling is carried out at from 450 to 600° C.(6) The manufacturing method of a steel sheet as set forth in (5), whichis characterized in that in addition to the foregoing composition, thecomposition further contains not more than 0.1% of Al in terms of % bymass.(7) The manufacturing method of a steel sheet as set forth in (5) or(6), which is characterized in that in addition to the foregoingcomposition, the composition further contains one or two or more membersselected from not more than 3.5% of Cr, not more than 0.7% of Mo, notmore than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005%of B in terms of % by mass.(8) The manufacturing method of a steel sheet as set forth in any one of(5) to (7), which is characterized in that the foregoing hot rolledsheet annealing is carried out at an annealing temperature of from 600to 750° C.

According to the invention, a steel sheet which is not only excellent inFB performance but also excellent in fabrication property after the FBworking can be easily and cheaply manufactured, thereby giving rise toremarkable effects in view of the industry. Also, according to theinvention, there are brought effects that a steel sheet excellent in FBperformance is provided; an end surface treatment after the FB workingis not necessary; a time of completion of manufacture can be shortened;the productivity is improved; and the manufacturing costs can bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph to show a relationship between an FB performance(surface roughness on a blanked surface) and an amount of a ferriteintergranular cementite.

FIG. 2 is a graph to show a relationship between an FB performance(surface roughness on a blanked surface) and a spheroidization ratio ofa cementite.

FIG. 3 is a graph to show a relationship between an FB performance(surface roughness on a blanked surface) and an average ferrite crystalgrain size.

FIG. 4 is an explanatory view to schematically show a measurement regionof surface roughness on a blanked surface after FB working.

BEST MODES FOR CARRYING OUT THE INVENTION

First of all, the reasons why the composition of the steel sheet of theinvention is limited are described. Incidentally, the “% by mass” in thecomposition is expressed merely as “%” unless otherwise indicated.

C: from 0.1 to0.5%

C is an element influencing the hardness after hot rolling annealing andquenching, and in the invention, C is required to be contained in anamount of 0.1% or more. When the content of C is less than 0.1%, thehardness required as automobile parts cannot be obtained. On the otherhand, since C is contained in a large amount exceeding 0.5%, the steelsheet becomes hard, an industrially sufficient mold life cannot beensured. For that reason, the content of C was limited to the range offrom 0.1 to 0.5%.

Si: not more than 0.5%

Si is an element not only acting as a deoxidizing agent but alsoincreasing the strength (hardness) due to solution hardening. However,when Si is contained in a large amount exceeding 0.5%, ferrite becomeshard, thereby reducing the FB performance. Also, when Si is contained inan amount exceeding 0.5%, a surface defect called as red scale isgenerated at the hot rolling stage. For that reason, the content of Siwas limited to not more than 0.5%. Incidentally, the content of Si ispreferably not more than 0.35%.

Mn: from 0.2 to 1.5%

Mn is an element not only increasing the strength of steel due tosolution hardening but also acting effectively in improving thequenching property. In order to obtain such an effect, it is desirablethat Mn is contained in an amount of 0.2% or more. However, when Mn iscontained excessively in an amount exceeding 1.5%, the solutionhardening becomes excessively strong so that the ferrite becomes hard,thereby reducing the FB performance. For that reason, the content of Mnwas limited to the range of from 0.2 to 1.5%. Incidentally, the contentof Mn is preferably from 0.2 to 1.0%, and more preferably from 0.6 to0.9%.

P: not more than 0.03%

Since P segregates on the grain boundary or the like and reduces theperformance, in the invention, it is desirable that P is reduced as faras possible. However, the content of P of up to 0.03% is tolerable. Forsuch a reason, the content of P was limited to not more than 0.03%.Incidentally, the content of P is preferably not more than 0.02%.

S: not more than 0.02%

S is an element which forms a sulfide such as MnS and exists as aninclusion in the steel, thereby reducing the FB performance, and it isdesirable that S is reduced as far as possible. However, the content ofS of up to 0.02% is tolerable. For such a reason, the content of S waslimited to not more than 0.02%. Incidentally, the content of S ispreferably not more than 0.01%.

The foregoing components are a basic composition. However, in theinvention, in addition to the foregoing basic composition, Al and/or oneor two or more members selected from Cr, Mo, Ni, Ti and B can becontained.

Al: not more than 0.1%

Al is an element not only acting as a deoxidizing agent but also bindingwith N to form AlN, thereby contributing to prevention of an austenitegrain from coarseness. When Al is contained together with B, Al fixes Nand B forms BN, thereby bringing an effect for preventing a reduction ofthe content of B effective for improving the quenching property. Sucheffects become remarkable when the content of Al is 0.02% or more.However, when the content of Al exceeds 0.1%, an index of cleanliness ofsteel is reduced. For that reason, when Al is contained, it ispreferable that the content of Al is limited to not more than 0.1%.Incidentally, the content of Al as an unavoidable impurity is not morethan 0.01%.

All of Cr, Mo, Ni, Ti and B are an element contributing to animprovement in quenching property and/or an improvement in resistance totemper softening and can be selected and contained as the need arises.

Cr: not more than 3.5%

Cr is an element effective for improving the quenching property. Inorder to obtain such an effect, it is preferable that Cr is contained inan amount of 0.1% or more. However, when the content of Cr exceeds 3.5%,not only the FB performance is reduced, but also an excessive increaseof the resistance to temper softening is brought. For that reason, whenCr is contained, it is preferable that the content of Cr is limited tonot more than 3.5%. Incidentally, the content of Cr is more preferablyfrom 0.2 to 1.5%.

Mo: not more than 0.7%

Mo is an element acting to effectively improve the quenching property.In order to obtain such an effect, it is preferable that Mo is containedin an amount of 0.05% or more. However, when the content of Mo exceeds0.7%, the steel becomes hard, thereby reducing the FB performance. Forthat reason, when Mo is contained, it is preferable that the content ofMo is limited to not more than 0.7%. Incidentally, the content of Mo ismore preferably from 0.1 to 0.3%.

Ni: not more than 3.5%

Ni is an element effective for improving the quenching property. Inorder to obtain such an effect, it is preferable that Ni is contained inan amount of 0.1% or more. However, when the content of Ni exceeds 3.5%,the steel becomes hard, thereby reducing the FB performance. For thatreason, when Ni is contained, it is preferable that the content of Ni islimited to not more than 3.5%. Incidentally, the content of Mo is morepreferably from 0.1 to 2.0%.

Ti: from 0.01 to 0.1%

Ti is easy to bind with N to form TiN and is an element effectivelyacting to prevent coarseness of a y grain at the time of quenching.Also, when Ti is contained together with B, since Ti reduces N whichforms BN, it has an effect for minimizing the addition amount of Bnecessary for improving the quenching property. In order to obtain sucheffects, it is required that the content of Ti is 0.01% or more. On theother hand, when the content of Ti exceeds 0.1%, the ferrite issubjected to precipitation strengthening due to precipitation of TiC orthe like and becomes hard, thereby reducing the mold life. For thatreason, when T is contained, it is preferable that the content of Ti islimited to the range of from 0.01 to 0.1%. Incidentally, the content ofTi is more preferably from 0.015 to 0.08%.

B: from 0.0005 to 0.005%

B is an element which segregates on an austenite grain boundary and whencontained in a trace amount, improves the quenching property. Inparticular, the case where B is compositely added together with Ti iseffective. In order to improve the quenching property, it is requiredthat the content of B is 0.0005% or more. On the other hand, even when Bis contained in an amount exceeding 0.005%, the effect is saturated andan effect that corresponds to the content cannot be expected, andtherefore, such is economically disadvantageous. For that reason, when Bis contained, it is preferable that the content of B is limited to therange of from 0.0005 to 0.005%. Incidentally, the content of B is morepreferably from 0.0008 to 0.004%.

The remainder other than the foregoing components is Fe and unavoidableimpurities. Incidentally, as the unavoidable impurities, for example,not more than 0.01% of N, not more than 0.01% of O and not more than0.1% of Cu are tolerable.

Next, the reasons why the structure of the steel sheet of the inventionis limited are described.

The steel sheet of the invention has a structure composed mainly offerrite and a cementite. The “structure composed mainly of ferrite and acementite” as referred to herein means a structure in which ferrite anda cementite account for 95% or more in terms of a volume ratio.

In the invention, the grain size of ferrite is from 1 to 10 μm in termsof an average crystal grain size. When the average ferrite crystal grainsize is less than 1 μm, not only the steel sheet is remarkably hardened,but also the cementite amount in ferrite grain increases, whereby thefabrication property such as hole expansion property after the FBworking as well as the FB performance and the mold life are reduced. Onthe other hand, when the grain size of ferrite exceeds 10 μm, though thesteel sheet is softened, thereby improving the mold life, the FBperformance is reduced as shown in FIG. 3. For that reason, the averageferrite crystal grain size was limited to the range of from 1 to 10 μm.Incidentally, the average ferrite crystal grain size is preferably from1 to 5 μm.

In the steel sheet of the invention, a spheroidization ratio of thecementite is 80% or more. When the spheroidization ratio is less than80%, not only the steel sheet becomes hard, but also the deformabilityis small and the FB performance is reduced. As shown in FIG. 2, when thespheroidization ratio is less than 80%, Rz ave exceeds 10 μm and becomeslarge, and the FB performance is abruptly reduced. For that reason, inorder to ensure a sufficient FB performance, the spheroidization ratioof a cementite was limited to 80% or more. Incidentally, in order tomake the spheroidization ratio large, since long-term annealing isrequired, the spheroidization ratio is preferably from 80 to 85%.

Also, in the steel sheet of the invention, an amount S_(gb) of a ferriteintergranular cementite is 40% or more. The amount S_(gb) of a ferriteintergranular cementite is a ratio of an occupied area of a cementitepresent on the ferrite crystal grain boundary to an occupied area of thewhole of cementites and is a value as defined by the followingexpression (1):

S _(gb)(%)={S _(on)/(S _(on) +S _(in))}×100  (1)

(wherein S_(on) represents a total occupied area of a cementite presenton the ferrite crystal grain boundary of the cementites present per unitarea; and S_(in) represents a total occupied area of a cementite presentin a ferrite grain of the cementites present per unit area.) When theamount S_(gb) of a ferrite intergranular cementite is less than 40%, theamount of the cementite present in the ferrite grain is large; Rz aveexceeds 10 μm and becomes large as shown in FIG. 1; and the FBperformance is abruptly reduced. It is considered that this is causeddue to the matter that when even a fine and spheroidized cementite ispresent in the ferrite grain, fine cracks are generated in the peripheryof the cementite at the time of FB working and connected to each other,thereby reducing the FB performance. It is also considered that whenfine cracks are generated in the periphery of the cementite at the timeof FB working and remain, these cracks are connected to each other inthe subsequent fabrication, leading to a reduction of the fabricationproperty. Also, when the cementite is present in the ferrite grain, theferrite grain itself becomes hard, thereby reducing the mold life. Forthat reason, in the invention, the amount S_(gb) of a ferriteintergranular cementite was limited to 40% or more. Incidentally, theamount S_(gb) of a ferrite intergranular cementite is preferably 50% ormore.

Also, in the steel sheet of the invention, it is preferable that thecementite present on the crystal grain boundary of ferrite has anaverage grain size of not more than 5 μm. This is because it has beennewly found that in the case where the amount S_(gb) of a ferriteintergranular cementite is 40% or more, with respect to the cementitepresent on the ferrite grain boundary, the smaller the particle size,the more improved the FB working and the larger the contribution to animprovement in mold life. Also, when the particle size of the cementite,in short-time heating in high-frequency quenching, it is possible toeasily dissolve the cementite in the austenite, whereby it is easy toensure a desired quenching hardness. For these reasons, it is preferablethat the average particle size of a cementite present on the ferritecrystal grain boundary is limited to not more than 5 μm.

Next, a preferred manufacturing method of the steel sheet of theinvention is described.

It is preferable that a molten steel having the foregoing composition ismolten by a common melting method using a converter or the like andformed into a raw steel material (slab) by a common casting method suchas a continuous casting method.

Subsequently, the obtained raw steel material is subjected to hotrolling to form a hot rolled sheet by heating and rolling.

The hot rolling is preferably a treatment in which a terminationtemperature of finish rolling is set up at from 800 to 950° C., aftercompletion of the finish rolling, cooling is carried out at an averagecooling rate of 50° C./s or more, the cooling is stopped at atemperature in the range of from 500 to 700° C., and coiling is carriedout at from 450 to 600° C. The hot rolling in the invention ischaracterized by adjusting the termination temperature of finish rollingand the subsequent cooling condition. Thus, a hot rolled steel sheethaving a substantially 100% pearlite structure is obtained.

Termination temperature of finish rolling: from 800 to 950° C.

It is preferable that the termination temperature of finish rolling is atemperature in the range of from 800 to 950° C., which is a terminationtemperature region of usual finish rolling. When the terminationtemperature of finish rolling exceeds 950° C. and becomes high, not onlya generated scale becomes thick so that the pickling property isreduced, but also a decarburized layer may possibly be formed in thesteel sheet surface layer. On the other hand, when the terminationtemperature of finish rolling is lower than 800° C., an increase in therolling load becomes remarkable, and an excessive load against a rollingmill becomes problematic. For that reason, it is preferable that thetermination temperature of finish rolling is a temperature in the rangeof from 800 to 950° C.

Average cooling rate after completion of finish rolling: 50° C./s ormore

After completion of the finish rolling, cooling is carried out at anaverage cooling rate of 50° C./s or more. Incidentally, the subjectaverage cooling rate is an average cooling rate of from the terminationtemperature of finish rolling to a stopping temperature of the subjectcooling (forced cooling). When the average cooling rate is less than 50°C./s, cementite-free ferrite is formed during cooling, and the structureafter cooling is a heterogeneous structure of (ferrite+pearlite),whereby a homogeneous structure composed of substantially 100% pearlitecannot be ensured. When the hot rolled sheet structure is aheterogeneous structure of (ferrite+pearlite), whatever the subsequenthot rolled sheet annealing is devised, the amount of the cementitepresent in the grain increases, and the amount of the cementite presenton the grain boundary decreases. Thus, the FB performance is reduced.For these reasons, it is preferable that the average cooling rate aftercompletion of finish rolling is limited to 50° C./s or more.Incidentally, for the purpose of preventing the formation of bentonite,it is more preferable that the average cooling rate after completion offinish rolling is not more than 120° C./s.

Cooling stopping temperature: from 500 to 700° C.

It is preferable that a temperature at which the foregoing cooling(forced cooling) is stopped is from 500 to 700° C. When the coolingstopping temperature is lower than 500° C., there are caused problems inoperation such as a problem that hard bentonite or martensite is formed,whereby the hot rolled sheet annealing takes a long time; and thegeneration of a crack at the time of coiling. On the other hand, whenthe cooling stopping temperature exceeds 700° C., and becomes high,since a ferrite transformation noise is present in the vicinity of 700°C., ferrite is formed during standing for cooling after stopping ofcooling, whereby a homogeneous structure composed of substantially 100%pearlite cannot be ensured. From these matters, it is preferable thatthe cooling stopping temperature is limited to a temperature in therange of form 500 to 700° C. Incidentally, the cooling stoppingtemperature is more preferably from 500 to 650° C., and furtherpreferably from 500 to 600° C.

After stopping the cooling, the hot rolled sheet is immediately coiledin a coil state. The coiling temperature is preferably from 450 to 600°C., and more preferably from 500 to 600° C.

When the coiling temperature is lower than 450° C., a crack is formed inthe steel sheet at the time of coiling, resulting in a problem inoperation. On the other hand, where the coiling temperature exceeds 600°C., there is a problem that ferrite is formed during the coiling.

The thus obtained hot rolled sheet (hot rolled steel sheet) is thensubjected to removal of an oxidized scale of the surface by pickling orshot blasting and subsequently to hot rolled sheet annealing. Byapplying proper hot rolled sheet annealing to the hot rolled sheethaving a substantially 100% pearlite structure, not only thespheroidization of a cementite is accelerated, but also the grain growthof ferrite is inhibited, whereby a large amount of the cementite can bemade present on the ferrite crystal grain boundary.

Incidentally, in the hot rolled sheet annealing, the annealingtemperature is a temperature in the range of from 600 to 750° C. Whenthe annealing temperature is lower than 600° C., spheroidization of thecementite cannot be sufficiently achieved. On the other hand, where theannealing temperature exceeds 750° C. and becomes high, pearlite isregenerated during cooling, and the fine blanking performance and otherfabrication property are reduced. Incidentally, though a holding time ofthe hot rolled sheet annealing is not required to be particularlylimited, in order to sufficiently spheroidize the cementite, it ispreferable that the holding time is 8 hours or more. Also, when itexceeds 80 hours, since the ferrite grain becomes excessively coarse,the holding time is preferably not more than 80 hours.

EXAMPLES

A raw steel material (slab) having a composition as shown in Table 1 wassubjected to hot rolling and hot rolled sheet annealing as shown inTable 2, thereby forming a hot rolled steel sheet (thickness: 4.3 mm).

The obtained hot rolled steel sheet was examined with respect to thestructure, FB performance and stretch flanging property after the FBperformance. The examination methods are as follows.

(1) Structure:

A specimen for structure observation was collected from the obtainedsteel sheet. A cross section parallel to a rolling direction of thespecimen was polished and corroded with nital; and with respect to aposition of ¼ of the sheet thickness, a metallurgical structure wasobserved (field number: 30 places) by a scanning electron microscope(SEM) (magnification, ferrite: 1,000 times, cementite: 3,000 times); anda volume ratio of ferrite and a cementite, a ferrite grain size, aspheroidization ratio of a cementite, an amount of ferrite intergranularcementite and an average particle size of a cementite on the ferritegrain boundary were measured.

With respect to the volume ratio of ferrite and a cementite, themetallurgical structure was observed (field number: 30 places) by SEM(magnification: 3,000 times); an. area ratio obtained by dividing anarea resulting from summing up an area of ferrite and an area of acementite by a total field area; and this value was judged as a volumeratio of ferrite and a cementite.

With respect to the ferrite grain size, an area of each ferrite grainwas measured, and a circle-corresponding size was determined from theresulting area and defined as a grain size of each ferrite grain. Thethus obtained respective ferrite grain sizes were arithmeticallyaveraged, and its value was defined as a ferrite average grain size ofthat steel sheet.

With respect to the spheroidization ratio of a cementite, a maximumlength a and a minimum length b of each cementite were determined ineach field (field number: 30 pieces) of the structure observation(magnification: 3,000 times) by using an image analyzer; its ratio a/bwas computed; and the number of cementite grains of a/b with not morethan 3 was expressed by a proportion (%) against the total number ofmeasured cementites, thereby defining it as a spheroidization ration (%)of cementite.

With respect to the amount of (S_(gb)) of a ferrite intergranularcementite, in each field (field number: 30 pieces) of the structureobservation (magnification: 3,000 times), a cementite present on theferrite grain boundary and a cementite present in the ferrite grain werediscriminated from each other; an occupied area S_(on) of a cementitepresent on the ferrite grain boundary and occupied area S_(in) of acementite present in the ferrite grain were measured by using an imageanalyzer; and an amount (S_(gb)) of a ferrite intergranular cementitewas computed according to the following expression (1).

S _(gb)(%)={S _(on)/(S _(on) +S _(in))}100  (1)

Also, with respect to each cementite present on the ferrite grainboundary, a diameter passing through two points on the periphery of thecementite and a center of gravity of a corresponding oval of thecementite (an oval having the same area as the cementite and having aprimary moment and a secondary moment equal to each other) was measuredat every 2° to determine a circle-corresponding size, thereby definingit as a grain size of each cementite. The thus obtained respectivecementite particle sizes were averaged, and its value was defined as acementite average particle size in ferrite grain.

(2) FB performance:

A specimen (size: 100×80 mm) was collected from the obtained steel sheetand subjected to an FB test. The FB test was carried out by blanking asample having a size of 60 mm×40 mm (corner radius R: 10 mm) from thespecimen by using a 110 t hydraulic press machine under a lubriciouscondition of a tool-to-tool clearance of 0.060 mm (1.5% of the sheetthickness) and a working pressure of 8.5 tons. With respect to an endsurface (blanked surface) of the blanked sample, a surface roughness(ten-point average roughness Rz) was measured, thereby evaluating the FBperformance. Incidentally, with respect to the specimen, in order toeliminate influences of a deviation in sheet thickness against theclearance, the both surfaces were equally ground in advance, therebyregulating the sheet thickness at 4.0+0.010. mm.

That is, with respect to the measurement of the surface roughness, asillustrated in FIG. 4, in each of four end surfaces (sheet thicknesssurfaces) other than R parts, a region within a range of from 0.5 mm to3.9 mm of the surface in the punch side in the sheet thickness directionand 10 mm in parallel to the surface (X direction) was scanned 35 timesat a pitch of 100 μm in the sheet thickness direction (t direction) byusing a contact probe profilometer, and a surface roughness Rz in eachscanning line was measured according to JIS B 0601-1994. Furthermore,with respect to the surface roughness Rz on the measured surface, Rzs inthe respective scanning lines were summed up, and an average valuethereof was employed. The four end surfaces were measured in the samemethod as described above, and an average surface roughness Rz ave (μm)defined according to the following expression was computed.

Rz ave=(Rz 1+Rz 2+Rz 3+Rz 4)/4

(wherein Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface.)

Also, the life of the used tool (mold) was evaluated. A surfaceroughness (ten-point average roughness Rz) of the sample end surface(blanked surface) at the point of time when the number of blanking inthe FB working reached 30,000 times was measured, thereby evaluating themold life. Incidentally, the measurement method the surface roughnesswas the same as described above. The case where the average surfaceroughness Rz ave of the sample end surface is not more than 10 μm isdefined as “O”; the case where it is more than 10 μm and not more than16 μm was defined as “Δ”; and the case where it is more than 16 μm wasdefined as “x”.

(3) Stretch flanging property after FB working:

A specimen (size: 100 mm×100 mm) was blanked from the obtained steelsheet by FB working, thereby examining a stretch flanging property.Incidentally, the FB working was carried out under a lubriciouscondition of a tool-to-tool clearance of 0.060 mm (1.5% of the sheetthickness) and a working pressure of 8.5 tons.

The stretch flanging property was evaluated by carrying out a holeexpansion test to determine a hole expansion ratio λ. The hole expansiontest was carried out by a method in which a punch hole of 10 mmφ (d₀)was blanked in a specimen and expanding the subject hole by a tool; ahole size d at the point of time when a through thickness crack wasgenerated in a flange of the punch hole was determined; and a holeexpansion ratio λ(%) as defined by the following expression wasdetermined.

λ(%)=(d−d ₀)/d ₀×100

The obtained results are shown in Table 2, too.

In all of the examples of the invention, the average surface roughnessRz ave on the blanked surface is not more than 10 μm; the FB performanceis excellent; the blanked surface at the time of 30,000 times inblanking number is smooth (evaluation: O); and a reduction in mold lifeis not acknowledged. Also, the examples of the invention are excellentin the stretch flanging property after FB working. Incidentally, thevolume ratio of ferrite and a cementite was confirmed in the foregoingmethod. As a result, in all of the examples of the invention, it wasconfirmed that the sum of volume ratio of the ferrite and cementite is95% or more, thereby forming a structure composed mainly of ferrite anda cementite. Also, the particle size of a cementite present on theferrite crystal grain boundary was confirmed by the foregoing method. Asa result, in all of the examples of the invention, the average particlesize was not more than 5 μm.

On the other hand, in the examples of comparison falling outside thescope of the invention, the surface roughness Rz on the blanked surfaceexceeds 10 μm and becomes coarse, whereby a reduction of the FBperformance is confirmed, the mold life is reduced, and the stretchflanging property is reduced. Incidentally, in the steel sheet No. 15,since a crack was generated at the time of coiling, the hot rolled sheetannealing and subsequent treatments were not carried out.

TABLE 1 Steel Chemical components (% by mass) No. C Si Mn P S Al N Cr MoNi Ti B Remark A 020 018   0.83 0.011 0.006 0.044 0.0035 — — — — —Invention B 0.35 0.18 0.79 0.014 0.006 0.028 0.0038 — — — — — InventionC 0.44 0.22 0.76 0.012 0.007 0.021 0.0042 — — — — — Invention D 0.200.17 0.74 0.017 0.009 0.025 0.0028 1.21 — — — — Invention E 0.16 0.170.88 0.012 0.008 0.024 0.0034 — 0.27 — — — Invention F 0.21 0.21 0.730.014 0.005 0.022 0.0031 — — 1.48 — — Invention G 0.19 0.16 0.74 0.0150.007 0.023 0.0029 — — — 0.04 — Invention H 0.21 0.18 0.73 0.014 0.0070.029 0.0044 — — — — 0.0024 Invention I 0.23 0.22 0.81 0.012 0.006 0.0350.0035 — — — 0.02 0.0016 Invention J 0.22 0.23 0.69 0.017 0.006 0.0260.0037 0.79 0.41 0.72 — — Invention K 0.21 0.24 0.71 0.015 0.006 0.0250.0042 0.78 0.28 1.23 0.02 0.0019 Invention L 0.34 0.65 0.83 0.015 0.0060.017 0.0044 — — — — — Comparison M 0.36 0.23 1.67 0.014 0.008 0.0220.0033 — — — — — Comparison N 0.36 0.18 0.72 0.011 0.006 — 0.0042 — — —— — Invention O 0.35 0.05 0.75 0.013 0.007 0.031 0.0040 — — — — —Invention P 0.22 0.20 0.70 0.013 0.006 — 0.0038 0.80 0.25 1.05 — —Invention

TABLE 2 Hot rolling condition Hot rolled sheet Structure TerminationCooling annealing condition Average Steel temperature Cooling stoppingCoiling Annealing Holding Sheet ferrite sheet Steel of finish ratetemperature temperature temperature time thickness grain size No. No.rolling (° C.) (° C./s) (° C.) (° C.) (° C.) (hr) (mm) (μm) 1 A 865  70620 570 710 30 4.3 9.5 2 B 860 100 630 550 720 40 4.3 9.2 3 B 865  30620 510 720 50 4.3 12.7 4 B 860 100 730 590 720 40 4.3 15.1 5 B 865 100400 350 710 50 4.3 — 6 C 840  80 620 580 710 40 4.3 8.8 7 D 855  80 650550 720 30 4.3 8.6 8 E 860 100 620 520 700 30 4.3 7.9 9 F 855  90 600480 730 40 4.3 9.8 10 G 860  80 630 550 710 50 4.3 5.7 11 H 860  80 640570 720 30 4.3 9.4 12 I 865  80 650 540 720 40 4.3 8.5 13 J 870 100 590500 720 40 4.3 6.9 14 K 860 120 630 570 730 60 4.3 5.2 15 L 860 100 640530 690 30 4.3 8.2 16 M 865 100 630 560 720 30 4.3 6.0 17 N 860  90 610510 720 40 4.3 7.4 18 O 855  80 600 540 710 30 4.3 8.3 19 P 865  80 580490 710 40 4.3 7.1 FB performance Performance after Steel StructureSurface roughness FB working Hole sheet Steel Spheroidization S_(gt), Rzave on blanked Mold expansion ratio λ No. No. ratio (%) (%) surface (μm)life (%) Remark 1 A 86 65 8 ∘ 105 Invention 2 B 92 68 8 ∘  93 Invention3 B 81 38 15 Δ  54 Comparison 4 B 74 35 17 x  47 Comparison 5 B — — — —— A crack was Comparison generated at the time of coiling. 6 C 90 67 8 ∘ 80 Invention 7 D 92 48 9 ∘ 121 Invention 8 E 85 56 6 ∘ 125 Invention 9F 87 66 7 ∘ 113 Invention 10 G 83 63 6 ∘ 108 Invention 11 H 84 64 7 ∘ 94 Invention 12 I 86 70 9 ∘  96 Invention 13 J 93 68 8 ∘ 105 Invention14 K 90 62 8 ∘  90 Invention 15 L 82 54 14 Δ  49 A red scale Comparisonwas generated. 16 M 84 53 13 Δ  55 Comparison 17 N 92 58 9 ∘  75Invention 18 O 85 52 9 ∘  89 Invention 19 P 88 45 8 ∘  92 Invention

1-8. (canceled)
 9. A steel sheet excellent in fine blanking performancecomprising: a composition containing from 0.1 to 0.5% of C, not morethan 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% of P andnot more than 0.02% of S in terms of % by mass, with the remainder beingFe and unavoidable impurities and having a structure mainly composed offerrite and cementite, wherein said ferrite has an average grain size offrom 1 to 10 μm, said cementite has a spheroidization ratio of 80% ormore, and of said cementite, an amount S_(gb) of ferrite intergranularcementite which is an amount of a cementite present on a crystal grainboundary of ferrite and which is defined by the following expression (1)is 40% or more:S _(gb)(%)={S _(on)/(S _(on) +S _(in))}×100  (1) wherein S_(on)represents a total occupied area of cementite present on the ferritegrain boundary of the cementite present per unit area; and S_(in)represents a total occupied area of cementite present in a ferrite grainof the cementite present per unit area.
 10. The steel sheet according toclaim 9, wherein the cementite present on the crystal grain boundary ofsaid ferrite has an average particle size of not more than 5 μm.
 11. Thesteel sheet according to claim 9, wherein the composition furthercontains not more than 0.1% of Al in terms of % by mass.
 12. The steelsheet according to claim 9, wherein the composition further comprisesone or more members selected from the group consisting of not more than3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass.13. A method of manufacturing a steel sheet excellent in fine blankingperformance including successively applying hot rolling by heating androlling a raw steel material to form a hot rolled sheet and hot rolledsheet annealing by applying annealing to the subject hot rolled sheet,wherein the raw steel material is a composition comprising from 0.1 to0.5% of C, not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not morethan 0.03% of P and not more than 0.02% of S in terms of % by mass, withthe remainder being Fe and unavoidable impurities; and said hot rollingis a treatment in which a termination temperature of finish rolling isfrom 800 to 950° C., after completion of the subject finish rolling,cooling is carried out at an average cooling rate of 50° C./s or more,the subject cooling is stopped at a temperature in the range of from 500to 700° C., and coiling is carried out at from 450 to 600° C.
 14. Themethod according to claim 13, wherein the composition further comprisesnot more than 0.1% of Al in terms of % by mass.
 15. The method accordingto claim 13, wherein the composition further comprises one or moremembers selected from the group consisting of not more than 3.5% of Cr,not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% ofTi and from 0.0005 to 0.005% of B in terms of % by mass.
 16. The methodaccording to claim 14, wherein the composition further comprises one ormore members selected from the group consisting of not more than 3.5% ofCr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass.
 17. Themethod according to claim 13, wherein the hot rolled sheet annealing iscarried out at an annealing temperature of from 600 to 750° C.
 18. Themethod according to claim 14, wherein the hot rolled sheet annealing iscarried out at an annealing temperature of from 600 to 750° C.
 19. Themethod according to claim 15, wherein the hot rolled sheet annealing iscarried out at an annealing temperature of from 600 to 750° C.
 20. Themethod according to claim 16, wherein the hot rolled sheet annealing iscarried out at an annealing temperature of from 600 to 750° C.